Sound recording and reproduction



Nov. 15, 1933. y F. MILLER 2,136,723

SOUND RECORDING AND REPRODUCTION Filed Sept. 25, 1955 V 5 Sheets-Sheet 1 T0 F'ILA ENT HEATE 5 FIG. 1.

INVENTOR. Burro/v FM/LLE BY M642? ATTORNEY Nov. .15, 1938. B. F. MILLER 2,136,723

SOUND RECORDING AND REPRODUCTION I Filed Sept. 25, 1955 3 Sheets-Sheet 2 Nov. 15 1938.

B. F. MILLER SOUND RECORDING AND REPRODUCTION s Sheets-Sheet 5 Filed Sept. 25, 1955 Patented Nov. 15, 1938 PATENT OFFICE 2,136,728 g SOUND RECORDING AND REPRODUCTION Burton F. Miller, Hollywood, Calif., assignor to United Research Corporation, Long Island City, N. Y., a corporation of Delaware Application September 25, 1935, Serial No. 42,116

. 11 Claims.

This invention relates to the recording and reproduction of sound and particularly to the recording of sound on light sensitive material to produce a diapositive record having varying light and dark areas or intensities and the reproduction of sound therefrom.

In recording, sound on film, the minimum volume level recordable and satisfactorily reproduceable, which may be considered the minimum variation in light and shade areas or intensities,

is that which is not less than the variations produced by the nature of the film, such as graininess and varying degrees of inherent transparency or the so-called film background noise 5"level. The upper or maximum variations recordable are those controlled by the width of the sound track which is of fixed dimensions or'the greatest variation obtainable between a transparent and opaque section. Both limits represent the go boundaries of sound levels recordable on film, the minimum being perhaps more troublesome, since the cause creating this minimum level is also present in reproduction at the time of no signal.

25 An object of the invention is to automatically increase the range of volume levels recordable upon the sound track of motion picture film and faithfully reproduceable therefrom, and thereby to obtain more effective noiseless recording than 30 is obtainable with noiseless recording systems now in common use.

This is accomplished according to one feature of the present invention by a method of and means for reducing the minimum volume level of the 5 original sounds which may be recorded and reproduced and thus increasing the range of recorded volumes. The method variable compression and expansion of the volume levels along a predetermined character- 40 istic between the minimum and maximum points which the record is capable of handling. That is, for the lower portion of the range of volume in reproduction, a certain volume expansion is provided, while this volume expansion may be dif- 45 ferent over the upper portion of the volume range.

The changes in volume expansion may be uniform otherwise. By compressed recording and expanded reproducing of the sound on the film, the record range of levels will be less than the 50 original range of levels but the reproduced range will be preferably equal to the original range. In recording, the lowest volume sounds are amplified to such an extent as to be recorded at a level equal to or above that corresponding to the film 55 background noise level during reproduction. As

involves the a consequence of this method of recording, the reproduced volume of the lower level sounds is reduced below that of the higher level sounds by an amount correspondingto that at which they were increased above their original levels, thus 5 likewise reducing the film background noise during periods of low level sound reproduction and during silent intervals. It will be shown later thatthe amount of this reduction is greater than that now commonly obtained with usual Well 10 known circuits.

When using this anti-ground noise system, there is automatically obtained a greater range of volumes within the limits of the film than formerly could be recorded and faithfully reproduced without the use of the invention.

.Another object of the invention, is to increase the volume range of reproduction and reduce ground noise from a sound record previously made according to any of the well known methods of noiseless recording, viz shuttering a variable area track, orbiasing a galvanometer mirror or light valve strings according to the amplitude of the recording. i

This is accomplished by using the reproducing portion of this system to reproduce standard or non-compressed "anti-ground noise sound records, the total noise reduction being the sum of the noise reduction of the record and of the reproducing system, the latter being determined by the maximum volume expansion employed. In this instance, however, the reproduced volume range is greater than that of the record range, this'belng accomplished by reducing the reproduced volumes of the low level sounds and consequently the film background noise. This, of course, entails expanding the upper level sounds abnormally, which produces dramatic and other presentation eifects of considerable value. When reproducing a non-compressed track, the incidental electrical .and mechanical noises which occurred inthe recording thereof and those originating in any portion of the reproducing equipment precedingthe expander are reduced in level by an amount equal to the totaldegree of volume expansion during low level reproduction or. at silent intervals, the reproducing system noises also being reduced when reproducing compressed records.

A further object of the invention is to more efliciently compress and/or expand sound volumes, when said sound volumes are in the form of electrical currents.

This is accomplished by similarly impeding the opposite half waves of the currents in accordance with the amplitude thereof.

Other objects and advantages of the invention will be obvious and the details of the invention more fully understood by reference to the following description read in conjunction with the accompanying drawings, in which:

Fig. 1 is a schematic drawing of a sound on film recording system involving the invention.

Fig. 2. is a schematic circuit diagram of a sound on film reproducing system embodying the invention.

Fig. 3 is a graph illustrating the relationship between the input and output of each of the systems in Figs. 1 and 2; and,

Figs. 4a and 4b are block diagrams showing the transmission level at various stages in the recording and reproducing systems.

Referring now to Fig. 1, the light modulation portion of the recording circuit includes a source of light 5 of constant intensity, a condensing lens 6, a light beam forming mask 1, a galvanometer 8, having a movable reflecting mirror, a light slit mask I0, an objective lens II and a film I2.

The operation of this portion of the system is well known in the art and operates in the usual manner; that is, the beam of light is moved longitudinally of the slit in mask Ill, thus varying the quantity of light striking the film. The galvanometer 8 is connected to the output of an amplifier I4, the input of which may be from terminals 9 connected to the output of tubes I6 and I1 which transmit sound currents therethrough serially with the line, or to terminals I3 connected to conductors I5 which eliminate tubes I6 and I1 from the circuit. The input of tubes I 6 and I1 is connected to the output of an amplifier 20, over terminals I8, the input thereof being connected to a microphone 2 I, or other sound-tocurrent translator. The output of amplifier 20 may also be connected to terminals IQ of conductors I5. Shunt resistances 22 and 3| function to match the impedances of input and output transformers with their connecting circuits.

The output of tubes I6 and I1 is impressed upon a three stage amplifier including vacuum tubes 23, 24 and 25, with the usual inter-connecting circuits. The output of this plural stage amplifier is impressed upon a full wave vacuum tube rectifier 21, 28. The rectified current is filtered by a low time constant filter 30, including resistance 302 and condenser elements 30I and 303 having values such that, together with terminating resistances 26 and 29, the time constant thereof is not greater than about .01 second and preferably as'low as .008 sec. The purpose of this low time constant is as follows: first, to insure that on increase of signal strength the audible amplification will reach its final value in so short an interval of time that no improper variation in signal level occurs, and second, to

insure that on decrease of signal strength the background film and system noise will not be unduly amplified afterthe signal has ceased. In order to insure that the rates of growth and decay of voltage across resistance 29 are substantially the same, resistance 26 having a value comparable to the minimum impedance of either of tubes 21 and 28, is connected in shunt to the filter input circuit between those tubes and the filter 30. This will be apparent from the fact that the time constant of the filter circuit, for currentpassing through the tubes 2! and 28 depends'in part on the impedance of those tubes. Current cannot pass in the reverse direction through those tubes and the resistance 26 is therefore added to minimize the effect of the tube impedances on the time constant. All of the vacuum tubes above mentioned have their elements polarized from a power rectifier unit 32 in the usual manner. This rectifier, however, has a variable tapped potentiometer resistance 33, the function of which will be explained hereinafter.

The output of filter 30 is connected in series With the grid-cathode circuit of impedance tubes I6 and I1 across the potentiometer terminating resistance 29 of the filter 30 and in series with a resistance 36. This connection provides tubes I6 and I"! with a grid bias and therefore a cathode. to-anode impedance determined by the amplitude of the current output of filter 30. The static value of this grid bias is determined by the potentiometer resistance 33, which is given a value in accordance with the characteristics of the tubes used and the function thereof. Filter condensers 34 and 35 are employed across the terminating resistance of the power supply circuit. By the use of resistance 36 of the desire-d value, any tendency for the grids to be driven positive by high signal levels will be counteracted by the voltage drop across this resistance caused by grid current flow. Thus a lower impedance limit for tubes I6 and I1 is thereby established. Tube I6 is shunted by a series arrangement of resistance 38 and a condenser 39, while tube I1 is shunted by a similar resistance 40 and condenser 4|. These elements are substantially non-reactive and function to provide an upper impedance value by preventing an effectively open circuit through tubes I6 and H.

The reproducing circuit of Fig. 2 is substantially identical to that of Fig. 1, except that the input circuit to the amplifier-rectifier circuit is now not connected to the output of tubes I6 and I! but is connected to their input circuit which comprises a photo-electric cell 45 or other light sensitive device upon which light from a constant intensity source 46 is varied in intensity by a sound record on film 41. The output of cell 45 is amplified by an amplifier 52 to which tubes 23, 24 and 25 are connected while the output of the system terminates in a loud speaker 49 or other sound reproducer connected to the output of an amplifier 50. The remainder of the circuit contains all the same elements of the circuit of Fig. 1 but has the connections of the output of filter 30 to the grids and cathodes of tubes I6 and I! now reversed while a difierent staticpotential is applied to tubes I6 and I I by potentiometer 33 as will be explained herein after. The same by-pass switches with conductors I5 are also employed in this circuit. Also in this system a unilateral device such as a pushpull amplifier having tubes 42 and 03 is inserted between the output of amplifier 52 and the input to impedance tubes I6 and I I to prevent unbalance impedance relations in the circuit of tubes I 6 and I! from reacting on the control circuit. The coupling between the tubes I6 and I1 and tubes 42 and43 is shown as a resistance-condenser circuit but these tubes may also be transformer coupled.

The impedance control of the above described recording and reproducing circuits, by means of impedance tubes I6 and I1, is the same except that the polarity of the rectified current from rectifier 30 is reversed in Fig. 2 from that in Fig. 1. The transmission characteristic of each circuit, or the relationship between input and outby the solid line plotted between input recording level and output recording level, and the reproducing characteristic shown by the dotted line plotted between the record level and reproduced level. Before describing the operation of the circuits, it is believed desirable to refer to Figs.

4a and 4b, in which the ranges of levels in each element of the circuit are shown. In Fig. 4a the input microphone 2| is shown feeding a microphone amplifier and mixer 55, the output of which has a level of approximately db. range extending from minus 40 db. to minus 80 db. A second amplifier 56 increases this range of levels from minus 40 db. to zero still maintaining the same original range. The degree of uniform amplification is arbitrary and is controlled by attenuation and the usual circuit factors. We then reach compressor 51, which includes the remainder of the circuit of Fig. 1 between the amplifiers 20 and [4, this circuit variably decreasing the upper and lower level limits so that the lowest level of minus 40 db. is now minus db. and the upper level is now at minus 20 db., the

difference constituting a range of 25 db. as com-.

pared with 40 db. The subsequent amplifier M then uniformly amplifies this range to levels between zero to minus 25 db., at which point it is impressed upon the galvanometer 8 to produce variations in.the light impinging on the film 12 in accordance therewith. This degree of amplification is also dependent on the load. It is thus seen that the original range of minus 40 to minus 80 db. is impressed on the film as a range extending from zero to minus 25 db., this range being within the limits of the film track, the lowest sound level, however, being above the film background level.

The reproduction of this record is shown in Fig. 45 wherein the reproducer 60 translates the record range of 25 db., but in view of losses in recording, this range now lies between minus db. and minus 75 db; However, the expander circuit 6| which is that part of Fig. 2 existing between amplifier 52 and amplifier 50,. extends this 25 db. range to the original range of 40 db. by reducing the higher volumes to 52 db. and the lower volumes to 92 db. A subsequent amplifier 52 y bring this range to any desired level such as plus 10 db. and minus 30 db. for impression on loud speakers 49, which translate the currents to sound energy. The range, therefore, impressed on the loud speakers, extends over 40 db., the same as the impressed range which originally extended from minus 40 to minus 80 db. at increased level limits dependent upon the output volume required for any particular area.

Reference to graph in Fig. 3 may further explain the general operation of the two circuits in which it will be noted that the relationship between the recording curve and reproducing curve appear as reflections of one another, about an axis which represents a one to one translation or amplification ratio between the input and output levels of the compressor and expander circuits. That is point a represents a minus 35 db. level for both input and output, this point also being the level of the film background noise. At this point, the signal to noise ratio is unity. This ,also means that no lower input levels may be recorded on the record than those between minus 35 and zero levels, otherwise they are masked by noise during reproduction. However, with the present invention, this input level may be extended to the point c,which is approximately minus db. since we are still not below the reproduced noise level limit of minus 35 db. This means that the recordable input range has been materially increased over that recordable with the systems now in use.

' Now in reproduction the point a is transferred to the point D on the reproducing curve, the input level now corresponding to an output level between zero and minus 55 db., this minus 55 db. level being below the film background noise level. This means that this background noise may not be reproduced at the loud speakers 49 with sufficient volume to be audible. In fact I have obtained with this invention of Figs. 1 and 2 a noise reduction of 20 db. better than the average obtained with standard noise reduction arrangements well known and in common use. Not only is the film background noise reduced but also the electrical and mechanical noises incident to reproduction are eliminated. In brief, therefore, the ratio between the variations in levels occurring in the original sound has been reduced for recording and the ratio between the variations in level occurring in the recorded sound is increased in reproduction.

The reproduction system of Fig. 2 may also be used to increase the volume range reproduced from a standard noise reduction sound track, this reproduction also following the broken line reproducing curve in Fig. 3. Thus, thistype of track will be reproduced without the present objectionable film background noise since the low levels reproduced are below the noise level. This means that the ratio of level variation is increased to bring the upper levels to zero db. By this variation, special effects such as louder pistol shots, siren noises, etc., will be accentuated.

To preventundue modulation of voice reproduction, the ratio of variation between levels may be changed between the upper and lower volume limits as shown by the curves in Fig. 3 thus producing a different ratio of volume variation at particular portions of the level range. This will be further described later.

The volume compression and expansion above described are accomplished by the circuits of Figs. 1 and 2 and particularly by varying the impedances of tubes I6 and H in accordance with the envelope of the soundcurrents impressed on the tubes I6 and I1. As mentioned above, these tubes constitute series impedances in the line inter-connecting amplifiers 52 and 50 in Fig. 2 and amplifiers l4 and 20 in Fig. 1. By the use of two of such tubes arranged in a parallel series arrangement, both halves of the sound currents are equally impeded, the amount of impedance being a function of the current of rectifier tubes 21 and 28, which is a function of the envelope of the sound currents. [While the relationship between the input and output current of tubes 21 and 28 is linear, the relationship between the input and output current for impedance tubes l6 and I1 is non-linear and varies in accordance with a power function dependent upon the relaand reproducing curves between output and input levels which represents the ratio of compression and expansion is controllable. In order to prevent the transmission impedance from droppin'g to an extremely low value approaching zero, a 100,000 ohm resistance 36is placed in the gird circuit, which will prevent the grids becoming positive, as any tendency to draw current in creases the drop across the resistance. This means, of course, that the impedance of tubes I6 and I! cannot be materially lowered beyond this point. The general range of variation may be determined and controlled by the position of potentiometer 33. In recording, a small bias is placed on the tubes, the rectified currents increasing this bias in accordance with their amplitudes, and thus varying the transmission loss through tubes l6 and H. In the reproducing circuit of Fig. 2, a high initial bias is employed, the rectified currents decreasing this bias and thus decreasing the transmission loss as the envelope of the sound currents increase.

Having thus described the invention, what is claimed as new and desired to secure by Letters Patent, is:

1. In a sound on film reproducing system, the combination of a carrier for a sound track having noise reduction, means for translating the modulations of said track into electrical currents corresponding thereto, an amplifier circuit, and a series-parallel impedance circuit connected in series intermediate said translator and said amplifier circuit for varying the amplitude transmission therebetween in a ratio different from that existing upon said carrier, said impedance circuit including the cathode-anode circuits of a pair of thermionic devices having a common grid circuit.

2. A sound recording system comprising a car rier for a sound record, said record having a certain percentage noise reduction, means for translating said record into electrical currents having volume levels corresponding thereto, an amplifier for amplifying said volume levels proportionately thereto, and a series-parallel impedance circuit intermediate said translator and said amplifier for varying the ratio between the volume levels of said transmitted electrical currents, said variations differing at different portions within the range of said volume levels, and said impedance circuit including fixed impedance elements in parallel.

3. A recording system in accordance with claim 2 in which said last mentioned means includes thermionic vacuum tubes in shunt to said respective fixed impedance elements, each of said tubes and associated elements transmitting opposite cycles of said currents with substantially equal variations.

4. In a sound reproducing system, the combination of a film carrying an anti-ground noise sound record, means for translating the variations of said record into corresponding electrical currents, an amplifier, a balanced impedance circuit inter-connecting said translator and said amplifier, said circuit including fixed impedances in parallel, a rectifier inter-connecting said translator and said impedance, and means included in said impedance responsive to the rectified current from said rectifier for variably changing the value of said transmission impedance, said change in impedance difiering for changes in rectified current of different values.

5. A' reproducing system in accordance with claim 4 in which said impedance includes a pair of thermionic devices in shunt to said fixed im-' pedances whose bias is controlled by said rectified currents and whose input and output circuits are in series with the cathode and anode of said devices.

6. A system of sound recording and reproduction comprising a source of current having a certain range of volume levels, a light sensitive medium for recording light variations in accordance with sound variations as a single track record, a light modulation system, an electrical circuit inter-connecting said source of electrical currents and said light modulating system, said means including a fixed impedance and variable impedance in parallel,- said variable impedance being a pair of thermionic vacuum tubes controlled by the envelope of said electrical currents for compressing the ratio between the original volume levels a predetermined amount, means for translating the single track record so produced into corresponding electrical currents, means for amplifying said currents, and means inter-connecting said translating means and said amplifying means for expanding the ratio between the volume levels of said record currents, the amount of expansion corresponding to the amount of compression produced during recording.

7. A variable impedance circuit comprising a thermionic device having cathode, grid and anode elements, input and output circuits of said device including said cathode and anode elements, a resistance shunt circuit to said cathode and anode, and a resistance in series with the grid to cathode circuit of said device, the impedance of said cathode to anode circuit varying in a manner and between limits determined by the values of said shunt resistance circuit and said grid to cathode circuit.

8. An electrical circuit comprising a transmission path, a control circuit for controlling the transmission of electrial currents over said path, a rectifier, a filter, said rectifier and filter being connected intermediate said control circuit and said transmission path, a filter output terminating resistance and a filter input terminating resistance element intermediate said rectifier and said filter, said filter input terminating resistance element having a value approximating the minimum impedance value of said rectifier to provide said filter with substantially equal rates of voltage growth and decay across said filter output terminating resistance.

9. An electrical circuit comprising a plurality of amplifiers, means having impedance and connecting said amplifiers, a control circuit for varying the impedance characteristics of said connecting means, a rectifier intermediate said control circuit and said connecting means, a filter in the output of said rectifier having a low time constant, said filter having an output terminating impedance, connections whereby the voltage across said terminating impedance is adapted to be applied to said transmission means, and a resistance intermediate said rectifier and said filter and in shunt thereto for providing the time constant of said filter with substantially equal rates of growth and decay of the voltage across said terminating impedance.

10. A system of sound recording comprising means for generating electrical currents corresponding to sound waves to be recorded, a recording device for translating electrical currents into modulated light, and an impedance circuit interconnecting said current generating means and recording device, said impedance circuit ineluding a pair of thermionic devices having respective cathode, grid and anode elements, the input and output circuits of said devices including said respective cathode and anode elements, resistance elements in shunt to each cathode and anode circuit and a resistance in series with a common grid circuit, the impedance of said cathode-anode circuits varying between the limits determined by the values of said shunt resistance elements and said grid circuit resistance.

11. A system of sound recording comprising means for generating electrical currents corresponding to sound to be recorded, a recording device for translating electrical: currents into modulated light, and impedance circuit interconnecting said current generating means and said recording device, said circuit comprising a pair of thermionic devices having a common grid circuit and having their cathode-anode circuits connected in series with each other and with said generating means and said recording device, and an amplifier-rectfier circuit having its input connected to the output of said thermionic devices and its output connected to the common grid circuit of said thermionic devices.

BURTON F. MILLER. 

